In Cline application Ser. No. 382,538, filed July 25, 1973, now U.S. Pat. No. 3,899,916 relating to "Recorder and Computer Type Brake Analyzer and Method," and assigned to the assignee of this application, a complete brake testing system is described in which a series of accurately controlled brake tests are systematically performed to provide brake performance data which is then evaluated to determine whether the brakes are malfunctioning. As explained in that application, after an initial rolling resistance test has established the unbraked resistance to vehicle wheel motion, three successive brake application strokes are designed to test first primarily for mechanical malfunctions in the brakes at the wheels, then primarily for hydraulic malfunctions in the hydraulic pressure transmitting system, and finally primarily for malfunctions at the frictional engaging surfaces. In each of the tests, the most significant indication of malfunction is imbalance between the left and right brakes, particularly those at the front of the vehicle but also, to a lesser extent, those at the rear of the vehicle. The absolute performance of each brake separately may also be significant, but to a lesser extent than relative values indicating left-to-right imbalance.
The following quotations from the specification of the Cline application, outline the steps which are useful in explaining the present invention:
"The first step of the procedure is to record the rolling resistance of the wheels with brakes released. This is necessary in order to establish the value of the resistance to rotation of the wheels with no brake engagement in order to provide a "base" for the charted readings."
"The second step of the test procedure is to apply the brakes gradually to a moderate magnitude of 50 to 70 pounds brake effort above the rolling resistance, hold this lever of effort for one or two seconds, and then release the brakes gradually. This step combines the tests of brake shoe engagement-lag, pre-hydraulic and brake shoe seating capability, and a brake release test."
"The third step of the test procedure comprises a rapid application of the brakes of 50 to 70 pounds above rolling resistance, followed by a quick release. This test evaluates the response and the other functioning of the actuating system and is basically a test to determine if there is a restriction to fluid flow in the system."
"The fourth and last step of the test procedure comprises a normal application of the brakes to an appropriate high level effort maintained for 5 to 6 seconds, followed by a normal release. This step of the test includes a test of the brake effort at comfort level application, brake effort at high level application, and constant imbalance, fade, and excursion at high level brake effort."
The fourth step referred to in the quotations is designed to check primarily the frictional subsystem of the vehicle brakes -- i.e., the energy-absorbing frictional components associated with each wheel.
The third step referred to in the quotations is designed to check primarily the hydraulic subsystem of the vehicle brakes -- i.e., the actuation signal from the driver to each wheel brake. (The hydraulic subsystem of the vehicle brake is not the same as, and should not be confused with, the hydraulic subsystem of the brake testing apparatus which constitutes the primary focus of this application).
The second step referred to in the quotations is designed to check primarily the mechanical subsystem of the vehicle brakes -- i.e., transformation of the hydraulic pressure signal into movement of the friction surfaces into contact.
In Clayton and Cline application Ser. No. 382,385 filed July 25, 1973, now U.S. Pat. No. 3,877,299 relating to "Brake Pedal Actuator" and assigned to the assignee of this application, a removable brake pedal actuator is disclosed which is placed on the vehicle floorboard and which uses a differential air pressure power unit to apply force to the brake pedal until a predetermined level of brake effort is reached, manual force applied by the operator being utilized to provide a reference force, or reaction point, which holds the brake pedal actuator in position. With the brake pedal actuator in position, air pressure can be applied under suitable control to cause the series of brake application strokes discussed above.
The desirability of improving the control of the brake application strokes in the testing series has led to the present invention. (The first, second and third brake application strokes correspond, respectively, to the second, third and fourth steps in the above quotations from application ser. no. 382,538).